By applying a voltage or a changing magnetic field to a nerve cell, it is possible to cause the nerve cell to “fire” during which the nerve cell depolarizes and then repolarizes.
In external magnetic stimulation, a strong alternating magnetic field is generated external to the body and is directed into the body. Within the body, the fluctuating magnetic field creates a current along the nerve cells that cause the cells to fire.
Such external systems require strong magnetic fields in order to penetrate into the body. However, as the magnetic fields increase in strength, the area affected by the magnetic fields also increases resulting in low resolution stimulus of the nerve cells. As a result, it is difficult to direct the external magnetic field to only a select number of nerve cells.
In deep brain stimulation, an electrode is inserted into the body and an electric field is generated in the target area to be stimulated. This electric field causes nerve cells to fire.